Question: $ { {-5} \times \left[\begin{array}{r}{1} \\ {-2} \\ {1}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-5}\times{1} \\ {-5}\times{-2} \\ {-5}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-5} \\ {10} \\ {-5}\end{array}\right]}$